Myocardial infarction (MI) results in left ventricular remodeling (e.g., ventricular hypertrophy, dilatation, and fibrosis). Fibrosis contributes to increased myocardial stiffening, impaired ventricular filling and function, and reduced cardiac output. Adenylyl cyclase (AC) expression and activity are reduced in animal models of heart failure. Stimulation of AC can inhibit extracellular matrix production in isolated cardiac fibroblasts; however, a role for reduced AC expression and activity in fibrosis associated with cardiac remodeling after chronic MI has never been determined. We tested the hypothesis that AC expression and activity are reduced in cardiac fibroblasts after chronic (18 wk) MI. Rats underwent coronary artery ligation or sham surgery (control), and echocardiography was used to assess left ventricular remodeling 1, 3, 5, 7, 10, 12, and 18 wk after surgery. Cardiac fibroblasts were isolated from the noninfarcted myocardium and compared for differences in AC activity and collagen synthesis. End-diastolic dimension was increased [control: 0.76 +/- 0.02 cm and MI: 1.0 +/- 0.02 cm (means +/- SE), P < 0.001] and fractional shortening was decreased (control: 44 +/- 2% and MI: 17 +/- 2%, P < 0.001) in MI compared with control rats. Basal and forskolin-stimulated cAMP production were decreased by 90% and 93%, respectively, and AC5/6 expression was decreased 39% in fibroblasts isolated from MI rats compared with sham controls. Serum-stimulated collagen production was increased twofold and forskolin-mediated inhibition of collagen synthesis was reduced in fibroblasts from MI rats compared with controls. Our data demonstrate that AC expression and activity are reduced and collagen production is increased in cardiac fibroblasts of rats after MI.